The active matrix type liquid crystal device, in which the non-linear active element (e.g. the switching transistor) is attached to each picture element, has become important and useful for pocketable color televisions and computer terminals, because the device provides pictures with high contrast and no cross-talk phenomenon by the switching function of the non-linear active element. For making such devices colorized, four processes have been proposed, i.e. (1) a guest and host effect process wherein a colorant is mixed in a liquid crystal, (2) a twisted nematic (TN) process using a colored polarizing panel, (3) an ECB (electrically controlled birefringence) process using the birefringence phenomenon of a liquid crystal by means of electric field, (4) a color filter process wherein a color filter layer which is colored with red, green and blue are formed in a liquid crystal cell and the liquid crystal layer is used as light shutter. In the four processes, the color filter process (4) is the most useful and important process, because the process can provide more contrast fullcolor pictures. In this color filter process, twisted nematic liquid crystal is employed.
The process (4) is classified into two groups in view of the arranging way of the two polarizing panels. One is normally black process wherein the device displays black color at no applied voltage (at OFF position), thus the polarizing panels being parallel with each other. The other is normally white process wherein the device displays white at no applied voltage (at off position), thus the polarizing panels being orthogonal with each other. Preferred is the normally white process in view of display characteristics, such as displayed picture contrast, color reproducibility and dependency on visual angle of displayed picture, if sufficiently high applied voltage can be ensured.
FIG. 4 (a) and (b) schematically show a liquid crystal display device to which a switching transistor q is attached. The switching transistors q and picture element electrodes c are formed at intersections of signal electrodes a and scanning electrodes b, and the elements a, b, c and q are all formed on a first substrate d. A counter electrode e is formed on a second substrate f, and a liquid crystal layer g is held between the first and second substrates d and f. One picture element is equivalent to an equivalent circuit and a driving voltage wave as respectively shown in FIG. 5 and FIG. 6 (a)-(c). In FIGS. 5 and 6, if the scanning signal V.sub.G which is applied to the scanning electrode b turns on the transistor q, the signal voltage V.sub.S which is applied to the signal electrode a is charged between the first and second substrates d and f, because the liquid crystal layer g is functioned as a condenser CL.sub.C. The charged voltage is kept between the substrates until the transistor q is turned on. Accordingly, a voltage V.sub.D is applied on the liquid crystal q at the same time as the static drive to display picture.
The above described FIG. 5 show ideal picture elements, but actually as shown in FIG. 7, the liquid crystal layer has a resistant component r which is equivalent to an electrical resistance RL.sub.C. It is believed that the resistance is caused by electroconductive materials, such as contaminations externally introduced into the liquid crystal or decomposed materials of the liquid crystal. Accordingly, the charged voltage V.sub.D, as shown in FIG. 8, discharges though the liquid crystal layer to result in decrease of voltage with time. The smaller the specific resistance, the larger the decrease of voltage In FIG. 9, a voltage wave applied to the liquid crystal layer is shown in case where PCH type liquid ##STR1## wherein R represents an alkyl group) having a positive dielectric anisotropy is employed. FIG. 9 (c) shows a V.sub.D voltage wave at 25 .degree. C. and FIG. 9 (d) shows a V.sub.D voltage at 80 .degree. C. This shows that the smaller the specific resistance of the liquid crystal, the smaller the effective voltage applied to the liquid crystal layer. In order to evaluate the decreasing amount of the effective voltage, a voltage retention (%) is introduced as a parameter. The voltage retention is defined effective value of actual V.sub.D voltage divided by effective value of V.sub.D voltage, when a resistant component of the liquid crystal are indefinitely high. The voltage retentions in FIG. 9 (c) and (d) are respectively 97% and 92%. As is known to the art, since liquid crystal exhibits accumulative response effective against voltage, the display characteristics of the liquid crystal depend on the effective voltage applied to the liquid crystal and therefore a decrease of the effective voltage reduces displayed picture contrast. In view of practical use, it is desired that the voltage retention is 98% or more at a maximum driving temperature. As is apparent from the above, the voltage retention depends on the product of capacity of the liquid crystal (C.sub.LC) and resistance R.sub.LC) (Time Constant), i.e. the product of specific dielectric constant L (.DELTA..epsilon.) and specific resistance. In order to enhance the voltage retention, it is required to increase the time constant of the liquid crystal, but B. Rieger et.al. have found that the time constant is tend to decrease as the specific dielectric constant increases (B. Rieger et.al. 18th. Int. Symp. on Liq. Crystal. Freiburg (1989)).
Accordingly, in order to increase the voltage retention, the specific dielectric constant of the liquid crystal should be lowered. This, however, means that the driving voltage of the liquid crystal becomes higher in view of the following relation; ##EQU1## wherein V.sub.th :threshold voltage of liquid crystal, K:elastic constant of liquid crystal. This causes many problems such as the increase of demand voltage of the circuit and the use of a circuit element which is resistant to high voltage. This raises more serious problems in case of projection type liquid crystal display devices and liquid crystal display devices for automobiles which are used at a high ambient temperature, as well as computer display of fine and high capacity.